DNA Test Result Question

I bred my purebred blue merle Aussie last summer with a black tri male and was very careful to keep her with me or confined during her heat cycle.

When she had her pups in June (6 live, 3 still born), I was stunned when some were white. I did some research and found out about Lethal Whites and that the male was probably a 'phantom' blue merle instead of a true black tri. Thankfully, the 2 white pups that survived have a lot of color around their ears and eyes and do not show any signs of hearing or sight loss.

When the pups went to their new homes, they all looked like Aussies, however, as they grew, the Aussie traits were lessened and they now look more like a Louisiana Cur dog! We had one of the pups DNA tested and the result came back 75% Aussie, 25% undetectable.

My question is this; if another male did get with my female wouldn't some of the pups look like full blooded Aussies and some look like the mix? Is it possible that the male, although registered, is not a purebred Aussie after all?

Any information you can give me to help me understand what happened with this litter is extremely appreciated.